Inkjet printing has become widely known and is most often implemented using thermal inkjet technology. Such technology forms characters and images on a medium, such as paper, by expelling droplets of ink in a controlled fashion so that the droplets land on the medium. The printer, itself, can be conceptualized as a mechanism for moving and placing the medium in a position such that the ink droplets can be placed on the medium, a printing cartridge which controls the flow of ink and expels droplets of ink to the medium, and appropriate hardware and software to position the medium and expel droplets so that a desired graphic is formed on the medium. A conventional print cartridge for an inkjet type printer includes an ink containment device and an ink-expelling apparatus or fluid ejection device, commonly known as a printhead, which heats and expels ink droplets in a controlled fashion.
The printhead is a laminate structure including a semiconductor or insulator base, a barrier material structure that is honeycombed with ink flow channels, and an orifice plate that is perforated with nozzles or orifices. The heating and expulsion mechanisms consist of a plurality of heater resistors, formed on the semiconductor or insulating substrate, and are associated with an ink-firing chamber and with one of the orifices in the orifice plate. Each of the heater resistors are connected to the controlling mechanism of the printer such that each of the resistors may be independently energized to quickly vaporize and to expel a droplet of ink.
During manufacture, ink with a carefully controlled concentration of dissolved air is sealed in the ink reservoir. When some types of ink reservoir are installed in a printer, the seal is broken to admit ambient air to the ink reservoir. Exposing of the ink to the ambient air causes the amount of air dissolved in the ink to increase over time. When additional air becomes dissolved in the ink stored in the reservoir, this air is released by the action of the firing mechanism in the firing chamber of the printhead. However, an excess of air accumulates as bubbles. Such bubbles can migrate from the firing chamber to other locations in the printhead where they can block the flow of ink in or to the printhead. Air bubbles that remain in the printhead can degrade the print quality, can cause a partially full print cartridge to appear empty, and can also cause ink to leak from the orifices when the printer is not printing.
Inkjet printing systems use pigment-based inks and dye-based inks. Pigment-based inks contain an ink vehicle and insoluble pigment particles often coated with a dispersant that enables the particles to remain suspended in the ink vehicle. Pigment-based inks tend to be more durable and permanent than dye-based inks. However, over long periods of storage of an inkjet pen containing pigment-based inks, gravitational effects on pigment particles and/or degradation of the dispersant can cause pigment settling or crashing, which can impede or completely block ink flow to the firing chambers and nozzles in the printhead. The result is poor performances, such as poor out-of-box performances (i.e. performance after shelf time) by the printhead and reduced image quality.
Furthermore, local evaporation of volatile components of ink, mostly water for aqueous inks and solvent for non-aqueous inks, results in pigment-ink vehicle separation (PIVS) or increased ink viscosity and viscous plug formation that prevents immediate printing. Printing systems tend to use thus massive ink spitting (ink wasting) before print job. This amount of ink sometimes exceeds multiple times the amount of ink used for image on paper.
Thus, although several suitable inkjet printheads are currently available, improvements thereto are desirable to obtain more durable and reliable printheads that will produce higher quality print images on print media surface.